Trying to build a Solar Hot Water System Inexpensively (so far)

[BrianSmith]

Well I've been working on building a low cost solar hot water system on and off for a couple of months.  I've spent about $150 on one 6 x 8 collector and a 55 gallon storage tank and insulation for it along with a little differential temperature controller to control the pump and record the temperature data.  I have a collector and storage tank built, with the plan to make a drain-back hot water heating system for less than $500.  I spent some time building and testing the hot water storage tank this weekend and got a couple of days of partly cloudy weather  solar collection data on how it did heating the water.  I will post some more data when we get some clear skies here in Alabama.

Here is a link to the project.

https://sites.google.com/site/brianssolarwaterheater/

Now I just have to get brave enough to break into the water pipes and make a heat exchanger to finish it up.....

[MattM]

If you ever do a project like this again you might consider buying a sheet of 26 gauge stainless and using copper channels to hold your PEX tubing.  Copper material is about $10/foot^2 for 16 ounce sheet materiel but you'd only need minimal strips to fasten down your PEX.  The copper would take the heat from the stainless - a good storage median - and transfer it to your PEX. The entire thing would still be paintable, although you'd want to use something thin and something that adheres to steel. The paint used for grills will if you keep it thin.  Copper is pricey, but it's EASY to form and transfer heat better than steel. Stainless is very rigid, withstands moisture and corrosion, and holds its form well whereas galvanized tends to wave under heating.  And these two materials are sanitary, resist corrosion, and are closely matched in valence.   

[BrianSmith]

I'm not completely in love with the way the aluminum flashing turned out, however I bought a roll for $25 and did the whole collector and have enough for about another 1/2 collector left.  My plan is to go with quantity (cheap and big) vs quality (smaller but more efficient).  Cash is a little tight, but sweat adn hard knocks I've got plenty of... 

[GaryGary]

Hi,
A couple thoughts on the collector fins:

- Steel or stainless steel fins are not a good way to go -- steel has only about 1/6th the thermal conductivity of aluminum, so it has to be 6 times thicker to work as well as the same thickness sheet of aluminum.  See the "Fin Efficiency" table on this page: http://www.builditsolar.com/Projects/WaterHeating/Fins/Fins.htm

- I think that the thin aluminum flashing is more challenging to use and get a fin that performs well.  A lot of it is tempered and hard to form, and its really to thin to get good thermal performance out of on a 6 inch wide fin.  I think that Kevin did come up with one good way to use the thin aluminum flashing material and get an efficient fin:  http://www.builditsolar.com/Projects/WaterHeating/Kevin1KSystem/KevinSystem.htm

- For people who don't want to go through all the make the fins process, Tom's fins are the cat's meow

The fin link above is a collection of quite a few ways people have worked out to do the fins. 

Gary

[BrianSmith]

Thanks for the links to better fin options.  I have seen Tom's website and pre built fins and I agree, they look very good. 

[mttrout]

Aluminum flashing can be formed using A  jig made from 2  2x4's spaced for your collector pipe diameter. measure the O.D.
of your pex tubing and find pipe of same diam. cut your flashing the needed length for collector then use it to mold your fins.
experiment on a few small lengths to determine how much is drawn into mold void  to get even either side of your pex,or copper.

[BrianSmith]

There are some pictures on the site of the mold I built I think.  I used some 1 x 2 screwed to a 2x4 piece and pressed the PEX directly into the aluminum flashing with a piece of wood behind it.   I was only pressing 1 ft pieces though so it wasn't too tough. 

[MattM]

Hi,
A couple thoughts on the collector fins:

- Steel or stainless steel fins are not a good way to go -- steel has only about 1/6th the thermal conductivity of aluminum, so it has to be 6 times thicker to work as well as the same thickness sheet of aluminum.  See the "Fin Efficiency" table on this page: http://www.builditsolar.com/Projects/WaterHeating/Fins/Fins.htm

Copper and aluminum are excellent at conducting heat.  Unfortunately they are not compatible and if you try to combine them they will corrode rapidly.  Stainless on the other hand is compatible with copper.  Its also very sanitary, whereas aluminum is a dirty material to work with once it gets exposed to weather.  The stainless can hold more heat than the aluminum and the copper strips can draw the heat off of it.  You can attack the copper to the stainless steel using hard solder and a mild acid or stay clean (R) for flux.

[GaryGary]

Hi,
A couple thoughts on the collector fins:

- Steel or stainless steel fins are not a good way to go -- steel has only about 1/6th the thermal conductivity of aluminum, so it has to be 6 times thicker to work as well as the same thickness sheet of aluminum.  See the "Fin Efficiency" table on this page: http://www.builditsolar.com/Projects/WaterHeating/Fins/Fins.htm

Copper and aluminum are excellent at conducting heat.  Unfortunately they are not compatible and if you try to combine them they will corrode rapidly.  Stainless on the other hand is compatible with copper.  Its also very sanitary, whereas aluminum is a dirty material to work with once it gets exposed to weather.  The stainless can hold more heat than the aluminum and the copper strips can draw the heat off of it.  You can attack the copper to the stainless steel using hard solder and a mild acid or stay clean (R) for flux.

Copper and aluminum are a ways away on the galvanic scale, so there is some potential for galvanic corrosion, but as a practical matter if you take some precautions you can build collectors with aluminum fins and copper tubing without having any problems.

This link talks about it some:
http://www.builditsolar.com/Experimental/CopperAlumCollector/GalvanicCorrosion.htm

There is a commercial collector that has been made for years by SunRay that uses aluminum fins with copper tubes, and the Maine Solar Energy Society has been doing workshops in which they make alum fin copper tube collectors -- they have been doing this for decades.

You don't want a material that "holds" heat -- adding thermal mass to collectors makes them slower to startup when the sun first hits them, and when the sun goes off the collector most of the heat stored in the collector mass is just lost out the glazing.  High conductivity is needed to efficiently get the heat from the fin to the tube, but a fins that "holds" heat is a disadvantage.

If you look down this page for the table of fin efficiencies, you can see what a big hit you take for steel fins:
http://www.builditsolar.com/Projects/WaterHeating/Fins/Fins.htm
You could overcome this by making the steel fin very thick, but this seems (to me) like an expensive and difficult way to go.

Gary

[MattM]

I wasn't trying to describe stainless steel fins, more to use as the backdrop in the collector.  The copper attached to the stainless would merely be heat channels to the focal point.  The solar collectors are never going to be 100% efficient, I was merely pointing out the affordable materials that are compatible.  Bonding aluminum and copper materials is not impossible, they just don't do it without a non-electrical conductive median, which means an insulator.  Not too many electrical insulators are going to excel at heat conduction, which means you are going to lose efficiency at the bond.  A sheet of stainless probably costs what he paid for the roll of aluminum.  Those rolls are typically .010 or thinner material,whereas I'm talking about 26 gauge (.018) stainless.  The specific gravity of aluminum is about 2.64, and the specific gravity of stainless is between 7.5 and 8.0 depending on the formula.  You just reached your six times point between the two.